Turbine engines provide energy for a wide range of uses. A typical turbine engine comprises a compressor, a combustor, a high-pressure turbine, and a low-pressure turbine. These components are typically contained within a casing which defines both an internal flow path of air through the engine and the external dimensions of the engine. In some applications, additional components are included to define all or part of the internal flow path.
Examples of turbine engines include turbofan, turbojet, turboshaft, and turboprop engines. In turbofan engines, a fan is typically located forward of the compressor, combustor, and turbines. The fan produces the majority of the engine's thrust during operation.
Axial flow engine fans are susceptible to aeroelastic instability, commonly called stall flutter or fan blade flutter, which occurs at part speed operating conditions near the stall boundary. Flutter is a self-excited vibration of a set of fan blades on a disk (rotor) and has the potential to escalate to a strength that can cause serious and sometimes catastrophic damage to turbine engine components.
One mechanism that can affect flutter is acoustic reflection within a nacelle which circumferentially surrounds the turbine fan. In some instances such acoustic reflection is in-phase with pressure oscillations experienced at the fan blade surface which exacerbates vibration at certain frequencies. Some acoustic reflections propagate axially (i.e. along the axis of rotation of the fan rotor) and circumferentially (i.e. about the axis of rotation of the fan rotor, such that any axially-forward propagation is in a spinning or corkscrew fashion).
To control the acoustic reflection propagation is an important design consideration in modern turbine engines because manipulating acoustic reflection propagation can reduce or eliminate flutter, which greatly reduces cost and frequency of maintenance of the fan and turbine engine, improves fan performance, and also greatly reduces the likelihood of serious or even catastrophic damage to turbine engine components.
While the present disclosure is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the present disclosure is not intended to be limited to the particular forms disclosed. Rather, the present disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the appended claims.